1. Field of the Invention
The present invention relates to an image coding method and apparatus to be used for compressing and transmitting or recording a digital image.
2. Description of the Prior Art
The transfer rate of digital images reaches several hundred Mbps to several Gbps, and there arise restrictions such as communication cost in transmitting and recording capacity cost in recording, and so on. Accordingly, hitherto there have been developed an image coding method and apparatus designed to lower the transfer rate while maintaining the degradation of the image quality to the minimum extent.
Hereinafter there will be described the image coding method of CCIR H.261 specification which is an example of the conventional image coding methods as mentioned above.
The objective input is a non-interlaced image which is called a CIF format. FIG. 10 shows a time-space diagram of the pixels of the input image. In the diagram, the abscissa shows the direction of time and the ordinate shows the vertical direction, i.e., line direction. In the diagram, the group of the pixels having a same time axis is called a CIF frame, and the coding is carried out in units of CIF frames.
The first CIF frame to be coded, i.e., the image of frame t, is subjected to intra-frame coding. That is to say, the image data is first divided into two dimensional blocks of 8.times.8 pixels and converted to transform coefficients in the unit of each two dimensional block by DCT (Discrete Cosine Transform). Next, the transform coefficients are quantized and sent out to the transmission line. In general, since the images have high correlations, and when they are subjected to DCT, the energies of the images having been subjected to DCT are concentrated on the transform coefficients corresponding to the low frequency components. Accordingly, by carrying out quantizing in such a manner as to roughly quantize the visually less significant high frequency components and finely quantize the low frequency components, which are the important components, it becomes possible to limit the degradation of image quality to a minimum extent and to reduce the data amount (cf CCITT Recommendation H.261, "Video coder for audiovisual services at px64 kbits/s", Geneva 1990)
However, the above-mentioned image coding method cannot efficiently code the interlaced scanned image inputs. At present, the generally used images are interlaced scanned. FIG. 11 is a time-spaced diagram of the pixels of the interlaced images. The pixels are arranged by interlacing to the time-shifted positions every vertical line. In the figure, in general, the group of the pixels having a same time axis is called a field, and a combination of the two fields having different time axes is called a frame. A frame is constituted by a field 1 and a field 2. To the input of FIG. 11, the conventional image coding method is applicable on a field by field basis, but in such case the correlations between fields, i.e., the vertical correlations, cannot be utilized, so that the coding efficiency is low. The field 1 and the field 2 have different time-space positions from each other, but in general have high correlations. For example, in the still image, the image does not change with time. When the points x, y and z in FIG. 11 are assumed, the correlation between the points x and y is higher than that between the points x and z. Therefore, next there is consideration to combine the field 1 with the field 2 to make a frame, and to carry out coding on the combined frame by frame basis. FIG. 12 is an example of having formed a frame by combining the fields, being expressed as a vertical and horizontal two-dimensional plane. In the figure, a triangle is panned from left to right. The frame formed by simply combining the fields has problematic points such that, in the case of the images having large movements, as in the hypotenuse of the triangle in the figure, there arises a part which shows degradation of correlations in the vertical direction, thereby providing a cause for the degradation of the image quality in coding.